Since the industrialization of titanium in the 1940s, its high specific strength, excellent corrosion resistance, non-magnetic properties, excellent weldability, and properties such as superconductivity, hydrogen storage, and memory have led to its widespread application in aerospace, military, marine development, petrochemicals, power generation, and superconductivity. It has earned the reputations of "all-purpose metal," "marine metal," "third metal," and "modern metal." As titanium's exceptional properties continue to be explored, its applications are expanding, poised to become the third structural metal after steel and aluminum.

 

This article reviews the progress in titanium research and application in aviation and aerospace in the United States, Russia, the United Kingdom, Japan, and China, providing valuable insights for the application and development of my country's titanium industry in these fields.

 

1 Titanium Alloy Raw Materials

 

Given its crucial role in defense, aviation, and high-tech fields, titanium has been highly valued by military powers such as the United States, Russia, the United Kingdom, and France, as well as Japan, and has been designated a strategically important structural metal for development in the 21st century. The development of titanium science and technology, including new alloys, new smelting techniques, and new application technologies, is undergoing rapid change. After nearly 40 years of ups and downs, China's titanium industry, with the support of the government, has made significant progress and established its own independent titanium industry system. Based on the 2000 production figures of 1,751 tons of titanium sponge and 2,206 tons of titanium processed materials, China produced 49,632 tons of titanium sponge in 2008, a 27.3-fold increase over the previous eight years. In 2008, China also produced 27,737 tons of titanium butadiene alloys, an 11.6-fold increase.

 

Due to the high cost of titanium alloy raw materials, 70% to 80% of titanium materials used abroad are used in the aviation and aerospace industries. my country's aviation and aerospace sectors also have a particularly high demand for titanium alloys. Currently, the proportion of titanium alloys used in advanced aircraft currently under development in my country is approximately 10% to 12%. Military aircraft use even higher amounts, at around 20% to 30%, while military aircraft engines use over 30%. The use of titanium in new rockets and missiles is also increasing.

 

2 Development and Application of Structural Titanium Alloys

 

As aircraft design concepts have gradually shifted from purely static strength to safety-life, damage-safety, and ultimately to modern damage-tolerance design concepts, advanced titanium alloys have also gradually developed toward damage-tolerant titanium alloys with high fracture toughness and low crack growth rates. Currently, developed countries abroad are at the forefront of the development and application of new damage-tolerant titanium alloys in advanced aircraft. In particular, alloys such as the medium-strength Ti-6Al-4V ELI and the high-strength Ti-6-2222S have been successfully implemented in new-generation aircraft such as the US F-22, F-35, and C-17, significantly improving their service life and combat effectiveness. With the advancement of aircraft design concepts, damage-tolerance design of titanium alloy structures has also begun to gain attention in my country. Since the 15th Five-Year Plan, my country has independently developed the TC4-DT medium-strength, high-toughness, damage-tolerant titanium alloy and the TC21 high-strength, high-toughness, damage-tolerant titanium alloy. Beta processing technology for these alloys has also been established, laying a foundation for the development of new aircraft materials. To meet the needs of titanium alloys for aviation and aerospace structures, my country has independently developed low-strength, high-toughness titanium alloys for wire (NbTi) and tubing (TAl8), as well as a series of ultra-high-strength titanium alloys with a strength range of 1300 MPa-2000 MPa (TB8, TBl9, and TB20). This has led to the formation of a new titanium alloy material system for aircraft structures with Chinese characteristics, laying the foundation for the application of a new generation of titanium alloys for aviation and aerospace structures.

 

Ti-6Al-4V (TC4) is a medium-strength α-β titanium alloy developed in the early 1960s. It possesses excellent comprehensive properties and is known as a versatile alloy. TC4 titanium alloy is the earliest and most widely used general-purpose titanium alloy for aviation and aerospace structures, including plates, bars, forgings, and castings. This alloy offers excellent welding and machinability. The fine-grained alloy exhibits superplasticity, enabling the fabrication of complex components using a combined superplastic forming/diffusion bonding (SPF/DB) process.

 

High-strength structural titanium alloys generally refer to alloys with a tensile strength exceeding 1000 MPa. Currently, the most advanced high-strength titanium alloys used in aircraft include the metastable β-type alloys Ti-15-3 and β321s, the near-β-type alloy Ti-1023, and the α-β two-phase titanium alloy BT22. Replacing the 30CrMnSiA high-strength structural steel commonly used in aircraft structures with high-strength structural titanium alloys can reduce weight by over 20%.

 

Ti-6Al-2Sn-2Zr-2Cr-2Mo (TC21) is a high-strength, high-toughness, and damage-tolerant two-phase titanium alloy developed in the 1970s. After thermomechanical treatment, this alloy exhibits high strength, excellent damage tolerance, and excellent fatigue crack growth resistance, making it suitable for the manufacture of high-strength, high-toughness load-bearing components. The addition of Si enables the alloy to maintain high strength at moderate temperatures, surpassing Ti-6Al-4V. This alloy sheet can be superplastically formed at room temperature.

 

Ti-10V-2Fe-3Al (TB6) is a high-strength, high-toughness, near-β-type titanium alloy developed in the late 1970s. This alloy offers high specific strength, excellent fracture toughness, a large through-hardening area, low anisotropy, excellent forging properties, and strong corrosion resistance. It combines many of the advantages of metastable β-type titanium alloys without sacrificing their solid solution properties, meeting the requirements of damage-tolerant design and high structural efficiency, reliability, and cost-effectiveness. Its maximum operating temperature is 320°C. The alloy's main products include bars, forgings, thick plates, and profiles. Solution and aging heat treatments achieve a well-balanced combination of strength, ductility, and fracture toughness, making it suitable for the manufacture of structural components requiring high strength and fracture toughness. Through thermomechanical treatment, high-temperature titanium alloys can achieve excellent toughness and low crack growth rates, making them suitable for structures requiring high fracture toughness.

 

3 Development and Application of High-Temperature Titanium Alloys

High-temperature titanium alloys, due to their excellent thermal strength and high specific strength, have found widespread application in aircraft engines. High-temperature titanium alloys are primarily used in aircraft engine fans and compressors, such as compressor disks, blades, navigation instruments, and connecting rings. Replacing nickel-based high-temperature alloys with titanium alloys can reduce compressor weight by 30% to 35%. Advanced foreign aircraft engines utilize 25% to 39% titanium. For example, the F100 engine uses titanium alloys for 25% of the total engine weight, the V2500 engine for 31%, and the F119 engine for 39%. The demand for high-performance aircraft engines has driven the development of high-temperature titanium alloys, leading to a gradual increase in operating temperatures, from 400°C in the 1950s, typically achieved with Ti-6Al-4V alloy, to 600°C, typically achieved with IM1834 alloy. Above 600°C, the sharp decline in creep resistance and high-temperature oxidation resistance are two major obstacles limiting the development of titanium alloys to higher temperatures. Therefore, 600°C is considered the "thermal barrier" temperature for titanium alloy development.

 

 

For many years, to meet the demands of high-performance aircraft engines, developed countries in the aviation industry, such as Europe, the United States, and Russia, have placed significant emphasis on the research and development of high-temperature titanium alloys, developing alloys suitable for use in temperatures between 350°C and 600°C. The former Soviet Union developed titanium alloys such as BT6, BT3-1, BT8, and BT9 in the late 1950s, and BT18 and BT25 alloys in the 1960s and 1970s. Subsequently, to enhance the performance and service life of these alloys, improved high-temperature titanium alloys such as BT18y, BT25y, BT8M, BT8-1, and BT8M-1 have been developed based on these existing alloys.

 

 

 

In recent years, BT36 titanium alloy has been developed for use in engines such as the HK8 and IIC90A. Similarly, the United States uses titanium alloys such as Ti64, Ti811, and Ti6242 in advanced engines such as the JT90 and F-110, respectively.

 

Russia's high-temperature titanium alloy development is highly sophisticated and mature, forming a complete titanium alloy system. Within a given temperature range, there are two or three high-temperature titanium alloy grades available. For example, BT8, BT9, and BT8-1 are suitable for use at 500°C; BT25 and BT25y are suitable for use at 550°C; and BT18y and BT36 are suitable for use at 600°C. Russia recommends BT25y for discs and rotor blades in aircraft engine high-pressure compressors operating at temperatures between 450 and 550°C, and BT18y for discs operating at temperatures between 550 and 600°C. Although BT36 has been developed, it appears to have received little attention. my country previously imported BT36 alloy discs and bars from Russia. Analysis revealed significant compositional segregation within these discs and bars, resulting in inadequate compositional uniformity. Furthermore, their high-temperature performance fell short of that of IM1834 alloy.

 

The UK has the most mature development of high-temperature titanium alloys, with its own independent system and a series of titanium alloy grades for use at different temperatures. To date, IM1685 alloy is the most widely used and most numerous high-temperature titanium alloy in British aeroengines, including in Rolls-Royes' RB211 series, RBl99, Adour, and M53 engines. IM1829 alloy is used in the high-pressure compressor of the RB211-535C engine. The rear three-stage disc, drum, and rear axle are integrated using electron beam welding, replacing the nickel-based alloy in the RB211-535C, reducing rotor weight by 30%. The successful development of IM1834 alloy has provided solid technical support for several high-performance engines. Although its development history is relatively recent, it has already been tested and applied in a variety of engines. For example, the Trent 700 (Turanda), a large civilian engine used on the Boeing 777, features IM1834 alloy for all its high-pressure compressor rotors, including the rotor drum and rear axle, welded together using electron beam welding. This makes the Trent 700 the first new civilian engine to utilize an all-titanium high-pressure compressor rotor, significantly reducing engine weight. The EJ200 engine also utilizes IM1834 alloy for its high-pressure compressor rotor. IM1834 is also currently used in Pratt & Whitney's PW350 engine.

 

 

The development of high-temperature titanium alloys in the United States is also relatively mature. Currently, the most widely used alloys in engines are Ti-6Al-4V and Ti-6242S. Ti-1100 alloy is based on the Ti-6242S alloy, but by adjusting the contents of Al, Sn, Mo, and Si, the maximum operating temperature of the alloy is raised to 600°C. It is understood that Ti-1100 alloy has been used in the manufacture of components such as the high-pressure compressor impeller and low-pressure turbine blades for the Lycoming T55-712 modified engine. 

 

The development of titanium alloys in my country has primarily been based on imitation. For example, TC11 alloy corresponds to BT9 alloy, while TA11, TA19, and TC17 correspond to American designations Ti-811, Ti-6242S, and Ti-17, respectively. In the past 20 years, my country has begun to pursue a strategy of imitation while also developing its own products. For example, the high-temperature titanium alloy TA12 (Ti-55) incorporates the rare earth element Nd. Ti-60 alloy, based on TAl2 alloy, has appropriately increased the contents of Al, Sn, and Si to further enhance the alloy's high-temperature creep properties and strength, allowing the alloy to be used at temperatures up to 600°C. Based on the British IM1829 alloy, China has added the rare earth element Gd to develop the 550°C high-temperature titanium alloy Ti-633G. Recently, Ti-1100 alloy has been developed by adding approximately 0.1% Y to the alloy, resulting in the designation Ti-600.

4 Development and Application of Low-Temperature Titanium Alloys Structural components used at low temperatures require good plasticity, low thermal conductivity, and excellent processing properties while maintaining a certain level of strength. Low-temperature structural materials used domestically and internationally primarily include stainless steel, aluminum alloys, titanium alloys, and nickel-based alloys. Titanium alloys exhibit excellent comprehensive properties at low temperatures and have been widely valued worldwide for many years. At low temperatures, the yield strength of titanium alloys increases significantly, reaching approximately 3 to 6 times that of austenitic stainless steel. However, the fracture toughness decreases with decreasing temperature, reaching approximately 0.25 to 0.5 of that of austenitic stainless steel. Because titanium alloys have a much lower density than stainless steel, low thermal conductivity, low expansion coefficient, and are non-magnetic at low temperatures, they are used as important low-temperature engineering materials in aerospace, superconductivity, and other fields. Like other body-centered cubic metals, β-titanium alloys with a bee structure at low temperatures have a high ductile-brittle transition temperature (TPR). Their ductility and toughness decrease with decreasing temperature, making them generally unsuitable for low-temperature use. α and near-α titanium alloys generally have very low TPRs and exhibit excellent ductility at low temperatures. Therefore, most of the currently recognized low-temperature titanium alloys internationally fall into this category. Among α-β titanium alloys, those with less β phase, such as Ti-6Al-4V ELI, can also function well in liquid hydrogen temperatures (22K). Pure titanium and α-titanium alloys such as Ti-5Al-2.5Sn ELI are ideal low-temperature structural materials in liquid helium temperatures (4.2K), but impurities outside the alloying components, particularly oxygen and iron, must be controlled. Increased iron and oxygen content increases the low-temperature brittleness of titanium. Furthermore, the addition of β-phase-stabilizing elements such as iron and manganese can easily lead to notch embrittlement. The former Soviet Union was once a world leader in the development and application of low-temperature titanium alloys. Its early α-titanium alloys, such as OT4, OT4-1, BT5-1KT, and TT-3BKT, have been widely used in aerospace rocket equipment. These alloys boast strengths reaching 1400 MPa at 2K, while maintaining elongation above 10%. Low-temperature titanium alloys developed and applied in the United States primarily include Ti-5Al-2.5Sn, Ti-8Al-1Mo-1V, and Ti-6Al-3Nb-2Zr.

 

 

China's development and application of low-temperature titanium alloys began later than the United States and Russia. After conducting low-temperature performance testing and application research on existing titanium alloys such as TA7, TC1, and TC4, my country developed titanium alloys suitable for low-temperature piping systems during the Ninth Five-Year Plan period. These alloy systems include Ti-Al-Zr, Ti-A1-Zr-Mo, Ti-A1L-Sn-Mo, and Ti-Al-Zr-Sn-Mo. 5 Development and Application of Titanium Alloy Fasteners The application of titanium alloy fasteners abroad has become very common, and various new fasteners are constantly emerging. The consumption of titanium alloy fasteners on a single large civil aircraft reaches hundreds of thousands of pieces. Under the same strength index, titanium fasteners are 70% lighter than steel. Moreover, the fatigue strength and sensitivity to stress concentration of titanium alloys are better than those of steel for similar purposes.